Much of the oil and gas produced today comes from accumulations in the pore spaces of reservoir rocks—e.g., sandstone, limestone, or dolomites. The amount of oil and gas contained in a unit volume of the reservoir is the product of its porosity and the hydrocarbon saturation. In addition to porosity and hydrocarbon saturation, the volume of the formation containing hydrocarbon is used to estimate total reserves. Knowledge of the thickness and the area of the reservoir may be used for computation of its volume. To evaluate the producibility of a reservoir, a determination may be made as to how easily fluid can flow through the pore system. This property of the formation rock, which depends upon the manner in which the pores are interconnected, is its permeability. Thus, petrophysical parameters which may be used to evaluate a reservoir are its porosity, hydrocarbon saturation, thickness, area, and permeability.
However, few of these petrophysical parameters can be measured directly. Instead, they are often derived or inferred from the measurement of other physical parameters of the formations. The other parameters may include, among others, resistivity, bulk density, hydrogen content (also known as hydrogen index), natural radioactivity, response to magnetization, spontaneous potential, etc.
Logging is the process of gathering and recording geological information from deep within the earth. A log (or well log) is a measurement versus depth or time, or both, of one or more physical quantities in or around a well. Wireline logs are taken downhole, transmitted through a wireline to surface and recorded there. Measurements-while-drilling (MWD) and logging-while-drilling (LWD) logs are also taken downhole. They may be transmitted to surface by mud pulses (transmitting pressure pulses in the mud), or else recorded downhole and retrieved later when the instrument is brought to surface, for example. A logging tool carries out measurements from which petrophysical properties of the earth in its vicinity can be derived. This process is often called well log analysis or formation evaluation.
Borehole logging may provide a cost effective and practical solution for identifying and characterizing hydrocarbon resources, such as heavy oil. Nonetheless, the log analysis of heavy oil reservoirs may be very challenging using typical logging measurements. For example, such measurements may not provide desired information about viscosity. For this, other measurement techniques such as nuclear magnetic resonance (NMR) logging may be used. Transforms may then be used to relate NMR relaxation times to fluid viscosity.